1. Field of the Invention
This invention relates generally to signal regenerators, and more specifically to such regenerators for a power-line communication system employing a power transmission line, having multiple conductors for each phase, as the signal propagation medium.
2. Description of the Prior Art
To provide centralized control and monitoring of a utility's power generation and distribution network a central computer communicates with remote terminals at each generating and switching station. At one time, power-line carrier systems were commonly used as communication channels for such power system control. Such communication systems transmit a long-wave modulated carrier signal over the three phase conductors of a power transmission line from one power substation to the next. The communication system includes a transmitter, a receiver, and associated coupling and impedance matching networks, connected at each terminal of the transmission line.
The transmitters operate at a carrier frequency in the range of 30 kHz to 300 kHz. Frequencies below 30 kHz are unusable due to the difficulty of building equipment to operate below this limit. Also, there is a substantial increase in received noise power below this limit. Frequencies greater than 300 kHz suffer substantial signal attenuation on the transmission line and increased radiation of the carrier signal, thereby possibly interfering with lone-wave radio services.
The primary source of noise at the carrier receiver is high-voltage corona on the energized transmission line. Transmitted-power levels, established according to attenuation of the line and level of corona noise at the receiver, are typically in the range of 1 to 10 watts. Thermal noise, which affects telephone or radio communication systems, is much smaller than corona noise and may be disregarded when calculating the performance of a power-line carrier channel.
Simple modulation schemes, i.e., on-off keying or frequency-shift keying are generally employed. Each modulated carrier signal typically occupies approximately 3 kHz of the frequency spectrum thus permitting, in theory, the multiplexing of approximately 90 individual modulated-carrier signals in the 30 kHz to 300 kHz band. Practical problems of adjacent channel interference, however, usually limit the number of signals to much fewer than 90 on any single power line.
Use of power-line carrier systems for power-system control has decreased in recent years. While they remain the simplest, least expensive, and most reliable of communication media available to the utility industry, the heavy use of power-line carrier for protective relaying communication and consequent congestion of the carrier spectrum have pushed utilities towards expensive microwave and telephone channels for system control and computer communication. The attractiveness of power-line carrier for data transmission in present-day circumstances would thus be greatly enhanced by devices which increase the number of available channels, and reduce interference among like channels on adjacent or parallel transmission lines. The present invention provides a device for attaining these objectives.
Although many power transmission lines comprise a single wire conductor for each phase, transmission lines operating at voltages above 230 kV use a bundle of spaced conductors to carry each phase current. A typical bundle consists of two or four conductors bundled together with conductive spacers to provide lower reactance and skin-effect losses than a single wire of the same total cross-sectional area. For power transmission, the bundled conductors in each phase are energized in the common mode.
In recent years the concept of using the bundled conductors of one phase in the differential mode for power-line carrier communication has evolved. For signal communication, a moderate level of insulation is placed between the conductors of each bundle, and a differential-mode communication signal is coupled to two conductors within the bundle, while continuing to use all conductors of the bundle in the common mode for electric power transmission. This scheme requires the use of split coupling capacitors, rated at the power-line voltage, to couple the carrier signal to and from the transmission line, and more costly insulating bundle spacers, in lieu of conducting spacers. Compared to conventional interphase signal propagation this intrabundle communication technique offers the advantages of increased bandwidth in each signal link and triplication of the number of available signal links, since each phase can be used as an independent channel. Also there is a virtual elimination of interference between channels on different phases of the same transmission line and between channels on adjacent transmission lines, and between the communication signal and radio services in the same frequency band. However, with intrabundle communication the signal attenuation increases noticeably on long lines and during foul weather, especially icing conditions. Therefore, it may be necessary to transmit a signal with an impractically high power level or to use one or more repeaters along the transmission line.
Repeaters and regenerators are frequently used in many types of long-distance communication systems to overcome signal degradation caused by noise and signal attenuation. Repeaters are used with analog modulation schemes; regenerators can be used only with baseband digital signals and pulse-code modulated signals.
In analog modulation a continuously varying carrier wave is modulated by a message signal. The modulated carrier assumes a wide range of values corresponding to the message signal. When the modulated carrier is adulterated by noise, a receiver cannot discern the exact value of the message at the time the interference occurred. To obtain adequate received signal strength, long-distance communication systems employing analog modulation, both free-space and cable, often use repeaters between terminals. These repeaters are well known in the art. With analog modulation, a repeater can do nothing more than simply amplify both the modulated carrier signal and the noise. Additional noise can also be introduced by the amplification process, causing the signal-to-noise ratio to progressively deteriorate at each repeater station.
With a pulse-code modulated signal or a baseband digital signal, the transmitted signal can have only a limited number of discrete signal values. If the amplitude or phase separation between these signal values is large compared to the noise perturbations, the receiver can determine the signal value despite noise interruptions, and accurately demodulate (or detect for baseband signals) the transmitted signal. Relying on this principle, a regenerator can therefore be utilized to demodulate (or detect), amplify, remodulate, and retransmit the signal, thereby producing a new signal free from noise (with the exception of detection errors arising during detection of the baseband signal). Like repeaters, regenerators are placed at critical locations along the transmission path. Use of a regenerator obviously prevents accumulation of noise interference and improves overall system performance.
As applied to communication systems operating on power transmission lines, prior art techniques teach the insertion of a repeater or regenerator in the signal path by decoupling the signal from the power line, processing the signal, and recoupling it to the power line for continued transmission. Such repeater or regenerator drops are expensive due to the cost of coupling capacitors rated at the power-line voltage and other power-line hardware.
Use of a prior art repeater or regenerator with the intrabundle communication scheme requires that the signal entering the repeater (or regenerator) be coupled from the high-voltage power line to the repeater input which is near ground potential. The amplified or regenerated signal must then be coupled back to the high-voltage power line. This requires two sets of costly split coupling capacitors rated for line voltage (i.e., one set at the repeater or regenerator input terminal and a second set at the output terminal). The present invention, a regenerator physically suspended from a power-line insulator and maintained at line potential, overcomes this expense. Also, during line deenergization the prior art repeaters and regenerators continue to operate. However, since line deenergization ends corona noise and the signal degradation it causes, it is unnecessary for the repeater or regenerator to continue operating under this condition. The present invention, therefore, incorporates a unique bypass arrangement whereby the regenerator is bypassed during line deenergization. These and other advantages of the present invention are discussed in detail below in the description of the preferred embodiment.